1. Field of the Invention
The present invention relates to an IF signal processing circuit formed in a semiconductor device in which many IF signal processing functions are integrated and the number of circuit components is reduced.
2. Description of the Related Art
A multisystem television receiver (hereinafter called a multivision receiver) which can receive a plurality of television broadcasting signals of different broadcasting systems has not been developed until recently.
Generally, there are various standards for a broadcasting system. For example, the TV broadcasting is classified into PAL system, SECAM system and NTSC system according to its color broadcasting system. The TV broadcasting is also classified into M system, B/G system, I system, D/K system, etc. according to its differences of carrier frequency and video signals. To receive the signals of a plurality of broadcasting systems in various countries all over the world, it is necessary for the processing circuit for intermediate frequency (hereinafter called IF) of the multivision receiver to process a video signal and a sound signal in accordance with an intercarrier frequency which is the difference between a video carrier frequency and a sound carrier frequency.
Especially, when a video signal subjected to amplitude modulation of negative polarity is processed in an IF signal processing circuit, it is possible to demodulate the TV signals of various broadcasting systems in a similar demodulating circuit, even though they are different in intercarrier frequency, frequency modulation degree (frequency deviation) and deemphasis time constant. Therefore, the multivision receiver which can demodulate TV signals of various broadcasting systems is now on the market.
The intercarrier frequency for the M system is 4.5 MHz, for the B/G system 5.5 MHz, for the I system 6.0 MHz, for the D/K system 6.5 MHz.
It is well known that since the frequency-modulated sound wave rides on the intercarrier frequency wave, the sound signal can be obtained by detecting the frequency-modulated signal on the intercarrier frequency.
Some multivision receivers have an auto mode function which can automatically determine the broadcasting system of input TV signal and can receive such signal without manual operation by the user.
When a general-purpose IF processing semiconductor device (hereinafter called IF IC) is used for the IF signal processing circuit of the multivision receiver which can receive TV signals of various broadcasting systems, it is typical to install a band-pass filter, a trap circuit, etc. (as external components) in the peripheral circuit of IC and to change over them by an external switching circuit.
In addition, (as external components), a sound amplifying circuit according to frequency modulation degree, deemphasis circuit according to preemphasis time constant, etc. are also installed in the IC peripheral circuit. These circuits are also changed over with the external switching circuit to demodulate the sound signal. It is typical to constitute such a peripheral circuit with a discrete component, etc.
FIG. 5 shows the block diagram of the conventional IF signal processing circuit.
In the FIG. 5, a tuner is omitted. The RF signals of various broadcasting systems are applied to such tuner (not shown). The tuner obtains an IF signal from the signal of predetermined channel of such RF signal and feeds it to the input terminal 50. The IF IC is shown in the dashed line frame. In this example, it is assumed that the signal of the NTSC or PAL system is applied to the input terminal 50.
The IF signal applied to the input terminal 50 is fed to a SAW filter 53 through the switching part 51 on the video signal side. The SAW filter 53 is a video band filter using the surface acoustic wave. The SAW filter 53 contains two filters having different signal bands in a single casing. The SAW filter 53 passes the video band signal of the IF signal of the NTSC or
system, but blocks any signal of other bands. The switching part 51 provided prior to the SAW filter 53 can be changed over according to the NTSC or PAL broadcasting system.
Similarly, on the sound signal side, the IF signal from the input terminal 50 is fed to the SAW filter 54 through the switching part 52. The SAW filter 54 also contains two filters with different signal bands in a single casing. The SAW filter 54 passes the sound band signal of the IF signal of the NTSC or PAL system, but blocks any signal of other bands.
The IF signal of the video band coming from the SAW filter 53 on the video signal side is amplified to a signal with sufficient amplitude at the amplifying circuit 55. Then the resulting signal is video-detected in the multiplying circuit 57. To the multiplying circuit 57, both video IF signal from the amplifying circuit 55 and the oscillation signal (having the same frequency as the video IF frequency) from the oscillator 58 are supplied. Both signals are multiplied with each other for synchronous detection and the base band video signal is obtained.
The IF signal of the sound band coming from the SAW filter 54 on the sound signal side is amplified to a signal with sufficient amplitude in the amplifying circuit 56. Then the resulting signal is intercarrier-demodulated in the multiplying circuit 59. To the multiplying circuit 59, both sound IF signal from the amplifying circuit 56 and oscillation signal (having the same frequency as the video IF frequency) from the oscillator 58 are supplied. Both signals are multiplied with each other, then the sound intercarrier signal is obtained as the difference between the oscillation signal and the sound IF signal. This is the frequency-modulated signal superimposed with the sound wave.
The outputs from these multiplying circuits 57, 59 are respectively fed to the video signal processing units (60-65) and to the sound signal processing units (66-74).
The video signal processing units (60-65) are equipped with a switching part 60 for the changeover according to each broadcasting system and the sound trap circuits and the equalizing circuits (61-64) of the characteristics corresponding to each broadcasting system. The purposes of such circuits are to remove the sound signal (unnecessary sound beat) of the different intercarrier of each broadcasting system and to achieve the same frequency characteristics (frequency-amplitude characteristics and frequency-phase characteristics) with respect to the different video signal frequency characteristics according to each broadcasting system. As shown in FIG. 5, the switching part 60 is installed both on the input and output sides of the sound trap circuits and the equalizing circuit (61-64). The switching parts 60 on both input and output sides are synchronously controlled by the switching controller 30. In the sound trap circuits contained in the sound trap circuits and the equalizing circuits (61-64), the sound trap frequency is changed over by the switching part 60 according to each broadcasting system (B/G, I and D/K of the PAL system and M of the NTSC system). The purpose of such sound trap circuits is to remove unnecessary sound beat (that is, sound intercarrier signal) in the video detection output from the multiplying circuit 57, caused by the sound IF signal which was not removed by the SAW filter 53. In the equalizing circuits contained in the sound trap circuits and equalizing circuits (61-64), the frequency-amplitude characteristics and the frequency-phase characteristics (group delay characteristics) of the video signal can be changed over by the switching part 60 according to each broadcasting system (B/G, I and D/K of the PAL system and M of the NTSC system). The purpose of such equalizing circuits is to achieve the same frequency-amplitude characteristics and the same frequency-phase characteristics (group delay characteristics) with respect to the different video signal frequency characteristics according to each broadcasting system. The video output signal is obtained at a terminal 65.
The sound signal processing units (66-74) are equipped with band-pass filters (67-70) corresponding to the intercarrier frequency of each broadcasting system. These band-pass filters (67-70) are changed over by the switching part 66 according to each broadcasting system. These band-pass filters (67-70) also feed only the signal within the intercarrier frequency band of each broadcasting system to the posterior SIF converting circuits (71-74). The frequency of the signal which passed through one of the band-pass filters (67-70) is converted by the SIF converting circuits (71-74).
These SIF converting circuit (71-74) is required, because the posterior FM demodulating circuit 76 of the quadrature type can detect the frequency-modulated signal of the single frequency alone. The purpose of such SIF converting circuits (71-74) is to keep the frequency of the output signal whose frequency was converted by the multiplying circuit 74 always constant by changing the oscillation frequency of the oscillator 73 (by changing over the crystals 71) according to the broadcasting system, even though the sound intercarrier signal frequency passing through the band-pass filters (67-70) corresponding to each broadcasting system is different according to the broadcasting system. The FM demodulating circuit 76 of quadrature type is the conventional typical circuit for this purpose.
The SIF converting circuit (71-74) comprises a multiplying circuit 74, an oscillator 73, a switching circuit 72 and a plurality (three in the figure) of crystals 71. However, in the SIF converting circuit, the multiplying circuit 74, the oscillator 73 and the switching circuit 72 are formed in an integrated circuit, excluding the crystals 71. In this case, a plurality of the crystals 71 alone are external components. To change over the oscillation frequency according to each broadcasting system, the oscillator 73 is equipped with the switching circuit 72 to change over a plurality (three in the figure) of crystals 71 as required.
The output signal from the SIF converting circuit (71-74) is fed to the FM demodulating circuit 76 through the band-pass filter 75 to demodulate the frequency-modulated sound signal.
To reproduce the sound signal according to the frequency modulation degree (frequency deviation) and preemphasis degree given to it on the transmission side in accordance with each broadcasting system, such sound signal is processed by the amplifying circuits and the deemphasis circuits 77A, 77B to be output from the output terminal 79.
In the above description, the amplifying circuits and the deemphasizing circuits 77A, 77B are for the NTSC and the PAL systems. These circuits 77A, 77B are set to the gain and the deemphasis level according to the NTSC and the PAL systems. The amplifying circuits and the deemphasizing circuits 77A, 77B are changed over by the switching circuit 78 installed on the output side according to the NTSC and the PAL systems. The switching circuit 78 is also changed over according to the control signal from the switching controller 30.
The frequency modulation degree of the NTSC (M) system is .+-.25 kHz, but such degree of the PAL (B/G, I and D/K) system is .+-.50 kHz. Therefore, the output level of the frequency-demodulated sound signal of the NTSC system is lower than that of the PAL system by 6 dB. It is necessary to compensate for such level down. For such purpose, the gain of each amplifying circuit contained in the amplifying circuits and the deemphasizing circuits 77A, 77B is changed over according to the NTSC and the PAL systems.
The preemphasis degree of the NTSC (M) system is 75 .mu.s, but that of the
(B/G, I and D/K) system is 50 .mu.s. Therefore, in the deemphasis circuit contained in the amplifying circuits and the deemphasizing circuits 77A, 77B, two deemphasis time constants are changed over for the frequency-demodulated sound signal according to the difference in the preemphasis of the NTSC and the PAL systems.
However, in the above-mentioned configuration, a lot of changeover switches are required for the changeover of various components according to each broadcasting system. Furthermore, in addition to the IF IC, other components such as and filters, switches, SIF converting circuits are required as the peripheral circuits. As a result, various problems such as increase in the number of components used, the number of assembly steps, and the circuit size occur.